Transparent conductors are thin conductive films coated on high-transmittance surfaces or substrates and can be manufactured to have surface conductivity while maintaining optical transparency. Transparent conducting materials are widely used as transparent electrodes for liquid crystal displays (LCDs), touch panels, organic light-emitting diodes (OLEDs), and solar cells, as anti-static layers and as electromagnetic wave shielding layers.
Because of their high electrical conductivity and high optical transparency, the most commonly used materials for such applications are doped metallic oxides, such as indium tin oxide (ITO), fluorine doped tin oxide (FTO), doped zinc oxide, aluminum-doped zinc oxide, and indium-doped cadmium oxide. However, use of ITO and other metallic oxides can be problematic because metallic oxide films are prone to cracking on flexible substrates due to the brittleness of the metallic oxide. Additionally, application of metallic oxides on substrates is an expensive process that requires deposition by a sputtering method in a highly specialized vacuum chamber.
Conductive polymers, which are organic polymers that conduct electricity, have also been used as optically transparent electric conductors. However, conductive polymers generally have lower conductivity and higher optical absorption compared to metallic oxide films. Additionally, conductive polymers suffer from lack of chemical and long-term stability.
Carbon nanotubes have also attracted interest as transparent conductors due to their mechanical and electrical properties. Although carbon nanotubes networks are both conducting and transparent, they have not been able to achieve the right combination of sheet conductivity and transparency to be competitive with the metallic oxides, such as ITO.
Graphene, which is a material composed of pure carbon, with atoms arranged in a regular hexagonal pattern, has been used to produce transparent conductors. Graphene is a single-atomic-layer of graphite. Similar to carbon nanotubes, graphene's sheet conductivity and transparency are not competitive, and the large-scale fabrication of grapheme is still under development.
Alternatives to metallic oxides, carbon based materials, and conductive polymers for conductive layers include conductive components such as metallic nano structures, including metal nanowires. Conductive layers formed of metal nanowires demonstrate transparency and conductivity equal to, if not superior to, those formed of metallic oxides. Metal nanowire films can be fabricated by cost-effective and scalable roll-to-roll coating processes and can be coated on glass or flexible substrates without the risk of cracking. Additionally, conductive layers using metal nanowires exhibit mechanical durability that metallic oxide transparent conducting materials do not. Therefore, transparent conductors formed of metal nanowires can be used in a number of applications, including on glass and in flexible display applications.
The transparency and conductivity of conductive layers fabricated from metal nanowires, however, depend upon the process by which the coatings are made. Typically, metal nanowires, such as, for example, silver nanowires, are grown via a polyol process. The metal nanowires are then purified and formulated into a coatable dispersion that is compatible with coating methods such as roll-to-roll slot die coating, spraying, meter-bar coating, or spin coating. By controlling the nanowire surface coverage, different sheet resistances can be produced. As illustrated in FIG. 1, when the silver nanowires form, there are no additional nanoparticles adhered to the surface of the nanowires. As-deposited films usually exhibit high resistance due to the insulation from surface-capping agents on nanowires and the loose contact between the nanowires. The conductivity of the metal nanowire film is largely influenced by wire-to-wire contact. In order to improve wire-to-wire contact for better electrical conductivity, heat treatment is usually required. The heat treatment typically occurs at 100° C. to 200° C. for 10 to 30 minutes. Without this heat treatment step, the resistance of the metal nanowire films is too high for electronic devices. While producing metal nanowire films with suitable resistance, such heat treatment, however, inhibits the use of metal nanowires on heat-sensitive substrates and adds an additional step to the fabrication process.
Japanese Patent Application Publication No. JP2009-94033 discloses a method of joining metallic nanowires and metallic nanoparticles by applying energy to a dispersion containing the metallic nanowires and metallic nanoparticles. JP2009-94033 states that the nanoparticles and nanowires are “joined” in a state in which the nanowires and nanoparticles are fused into a single continuous body electrically. JP2009-94033 distinguishes the fused nanowires and nanoparticles from when objects are simply in contact, stating that a loss of conductivity occurs due to contact resistance. JP2009-94033 describes the joining as “nanosoldering,” specifically applying laser energy from a Nd-YAG laser with light equivalent to the surface plasmon absorption wavelength of the metal nanoparticles. The lowest resistivity obtained in JP2009-94033 is 90 Ω/sq at 88% transmittance. This is close to the industry standard for Indium Tin Oxide (ITO) film (100 Ω/sq at 90% transmittance). This performance, however, was obtained with highly concentrated dispersions of silver nanowires (5%) and gold nanoparticles (5%), after heat treatment at 80° C. and prolonged laser irradiation. Table 1 of JP2009-94033 demonstrated that a 5% dispersion is more effective than a 0.5% dispersion in joining the nanoparticles to the nanowires. JP2009-94033 did not report data on haze, but it is known in the field that a more highly concentrated dispersion with nanostructures will result in higher haze to the film. Haze refers to the milky appearance of the surface, generated by discrete particles in the film that cause diffused light with low intensity adjacent to the main direction of reflection. Haze is not desired for touch panel applications. Thus the process described in JP2009-94033 is not suitable for making transparent conductors with low resistivity and low haze simultaneously. In addition, such a process requiring application of laser energy to nanosolder the nanoparticles to the nanowires would be costly and difficult to utilize in large scale manufacturing processes.
The article “Efficient Welding of Silver Nanowire Networks without Post-Processing,” Small, pp. 1-8, 2013 by Jaemin Lee, Inhwa Lee, Taek-Soo Kim and Jung-Yong Lee describes the formation of silver nanowire (AgNW) films and observes that the presence of a polymer coating on the surface of the nanowire, namely, polyvinylpyrrolidone (PVP), causes a critical problem of inhibiting conduction across the wires. Lee et al. note that the PVP present between the nanowires increases contact resistance and limits the sheet resistance of the AgNW network. Therefore, Lee et al. undertake steps to remove the layer of PVP a few nanometers thick on the AgNWs to facilitate electrical connection between the wires. Lee et al. use polar solvents to exfoliate the PVP that adheres to the wires by weak Van der Waals forces. Lee et al. indicate that ethylene glycol, glycerol, or alcohol together with centrifugation is needed to remove the PVP layer. Lee et al. reduced the PVP layer from 4 nm to 0.5 nm by using a washing and filtering with methanol. Such washing techniques are time and labor intensive, limiting the application of such a process to large scale industrial production. Lee et al. demonstrated that improved wire-to-wire contact can be obtained via spray deposition at an optimized condition. However, this is not adaptable for conventional large-scale deposition techniques such as roll-to-roll printing. Lee et al. do not discuss the interaction of nanoparticles and nanowires.
Accordingly, there is a need for a method of forming transparent conductors with acceptable optical and electrical properties that can be used with heat-sensitive substrates. It would be desirable to provide a method that did not require extensive washing, filtering, and/or application of radiation or heat to solder the wires together.